ASRJC Alternating Current

ANDERSON SERANGOON JUNIOR COLLEGE PHYSICS 9749 18-1 Additional Notes Topic 18: Alternating Current Content: A Characteristics of alternating currents B Rectification with a diode C The transformer Learning Outcomes: Candidates should be able to: (a) show an understanding of and use the terms period, frequency, peak value and root-mean-square (r.m.s.) value as applied to an alternating current or voltage (b) deduce that the mean power in a resistive load is half the maximum (peak) power for a sinusoidal alternating current (c) represent an alternating current or an alternating voltage by an equation of the form x = x0 sin ωt (d) distinguish between r.m.s. and peak values and recall and solve problems using the relationship 0 /2rmsII = for the sinusoidal case (e) show an understanding of the principle of o peration of a simple iron -cored transformer and recall and solve problems using / / /s p s p p sN N V V I I== for an ideal transformer (f) explain the use of a single diode for the half-wave rectification of an alternating current. A Characteristics of Alternating Currents Understanding the Nature of Scientific Knowledge During the ‘War of the Currents” in the late 1880s, Thomas Edison (1847 -1931) favoured direct current while George Westinghouse (1846-1914) argued for alternating current. Direct current at 110V used to be the standard for electricity distribution in United States in terms of safety and practicality. However, the use of alternating current eventually prevailed as it was more effective in minimising power loss with the use of transformers and high voltage in long -distance transmission. Historical competition between companies for electrical power distributio n and other factors resulted in different standards for the mains voltage in different countries. Having different voltages and frequencies between countries, and for some, within the country, can create confusion and result in extra costs for electrical appliances and adaptors. In Singapore, 230V at 50 Hz is used. In line with technological changes in the way the society uses electricity, there is ongoing research to continue to find ways to minimize power losses in electrical power transmission. It appear s that direct current now has an opportunity to dominate many mo dern electrical digital devices . The method that will eventually triumph in the future will be the one that results in the least power losses.

ANDERSON SERANGOON JUNIOR COLLEGE PHYSICS 9749 18-2 Additional Notes A.1 Introduction to Alternating Currents An Alternating Current is a current that varies periodically in direction. Direct Current Alternating Current Polarity of d.c. supply remains constant with time. Polarity of a.c. supply changes with time. Current is unidirectional, from positive to negative terminal. Current changes direction. Current may increase and decrease in magnitude